Using Associated Objects

Associated objects are useful when you want to add functionality to existing classes which requires holding state.

For example, adding a activity indicator to every UIView:

Objective-C Implementation

#import <objc/runtime.h>

static char ActivityIndicatorKey;

@implementation UIView (ActivityIndicator)

- (UIActivityIndicatorView *)activityIndicator {
    return (UIActivityIndicatorView *)objc_getAssociatedObject(self, &ActivityIndicatorKey);
}

- (void)setActivityIndicator: (UIActivityIndicatorView *)activityIndicator {
    objc_setAssociatedObject(self, &ActivityIndicatorKey, activityIndicator, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void)showActivityIndicator {

UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle: UIActivityIndicatorViewStyleGray];

[self setActivityIndicator:activityIndicator];

activityIndicator.center = self.center; activityIndicator.autoresizingMask = UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleBottomMargin;

[activityIndicator startAnimating];

[self addSubview: activityIndicator];

}

- (void)hideActivityIndicator {

UIActivityIndicatorView * activityIndicator = [self activityIndicator];

if (activityIndicator != nil) { [[self activityIndicator] removeFromSuperview]; }

}

@end

You can also access the Objective-C runtime through Swift:

Swift Code

extension UIView {

private struct AssociatedKeys { static var activityIndicator = “UIView.ActivityIndicatorView” }

private var activityIndicatorView: UIActivityIndicatorView? { get { return objc_getAssociatedObject(self, &AssociatedKeys.activityIndicator) as? UIActivityIndicatorView } set (activityIndicatorView) { objc_setAssociatedObject(self, &AssociatedKeys.activityIndicator, activityIndicatorView, .OBJC_ASSOCIATION_RETAIN_NONATOMIC) } }

func showActivityIndicator() { activityIndicatorView = UIActivityIndicatorView(activityIndicatorStyle: .gray) activityIndicatorView.center = center activityIndicatorView.autoresizingMask = [.flexibleLeftMargin, .flexibleRightMargin, .flexibleTopMargin, .flexibleBottomMargin]

activityIndicatorView.startAnimating()

addSubview(activityIndicatorView)

}

func hideActivityIndicator() { activityIndicatorView.removeFromSuperview() }

}